The present invention relates to a data transmission system utilizing a power line of a 3-phase alternating current. More specifically, the present invention relates to a data transmission system utilizing a power line of a 3-phase alternating current for transmission of data in synchronism with the cycles of alternating current of a power supply.
A system for transmission of data utilizing a power line of a single-phase alternating current has been conventionally proposed and put into practical use. In addition, such a data transmission system is extended to a system utilizing a power line of a 3-phase alternating current. For example, the system for controlling various indoor electric appliances by superposing a high frequency carrier as controlled data on an alternating current wave of a power supply by the use of an indoor power supply line has been put into practical use. In general, the transmission of data is performed in synchronism with the cycles of the alternating current of a power supply. More particularly, referring to FIG. 1, using as the reference a phase of an alternating current the zero voltage crossing (zero-cross point), the spacing between two zero-cross points is divided into four sections. The data is determined by a combination of carriers existing in these four sections. For example, the first spacing between zero-cross points in FIG. 1 indicates a start signal, the second and fourth spacings between zero-cross points indicates a data of "1" and the third spacing between zero-cross points indicates a data of "0". The control of a load can be achieved using the data for controlling the load, the data being structured by a plurality of these data of "1" and "0".
In the data transmission system extended to a system utilizing a power line of a 3-phase alternating current power, there exists a problem of synchronism between a transmitter and a receiver. More particularly, since the position of the zero-cross point is different for each phase R, S, T, of to FIG. 4, the data transmission cannot be made between the transmitter and the receiver which are connected to different phases, using as a reference the respective zero-cross point. For example, the data transmitted in synchronism with the zero-cross point of the R phase will be received as different data if the receiving of such data is synchronized with the zero-cross points of the S phase or the T phase. Accordingly, it becomes necessary to provide a synchronization of signals between a transmitter and a receiver in a data transmission system using a power line of a 3-phase alternating current. The prior art typically provides a synchronization of a signal between a transmitter and a receiver in a data transmission utilizing a power line of a 3-phase alternating current a disclosed in U.S. patent application Ser. No. 200,079, filed Oct. 24, 1980 by Yoshiharu Suzuki et al, assigned to the same assignee, Matsushita Electric Works, Ltd. In this prior art, prior to transmission of a control signal, a synchronization signal, one that is necessary for providing a synchronization of a signal between the transmitter and the receiver, is sent out. The synchronization signal is sent out in a particular predetermined code. On the receiver side, a synchronization of a signal to the transmitter side is achieved by receiving the synchronization signal with reference to the zero-cross point of its own receiver and detecting a shift of the phase with respect to the transmitter side based on the variation of the above described code of the received signal. However, in this prior art, the synchronization between the transmitter and the receiver set by a detection of the synchronization signal cannot be necessarily maintained until the subsequent control signal is completely received. This is because a synchronization circuit in the prior art is always in an enable state and thus, if the same code is a synchronization signal changed due to an influence of noise or the like which happens to exist in a control signal subsequent to the synchronization signal, the synchronization circuit is necessarily responsive to such signals. Accordingly, in such a case, a synchronized relation between a transmitter and a receiver can be released in the course of the data transmission. In such a case, the data transmission will be a failure.
Therefore, it is desired to provide a data transmission system utilizing a power line of a 3-phase alternating current in which, once a synchronization of signal between a transmitter and a receiver is set in response to a detection of a synchronization signal, the synchronized relation can be maintained until the data transmission is terminated. As one approach for structuring such a data transmission system, it may be considered that a signal for disabling a synchronization circuit is provided immediately after a synchronization signal, or a signal for enabling again the synchronization circuit is provided after a data transmission. However, such approach makes a transmitter and a receiver complicated, which is not economical.